When installing decoder and LED's I know to use a resistor rated at least 680 ohms, is it ok to use a resistor with a higher rating? I assume too high of rating will prevent the LED from lighting, so what is the acceptable ohm range for resistors used for LED's ?
I also noticed that resistors have watt ratings, what is the suggested watt rating when used for LED's?
Finally I know resistors have colored rings to indicate their value but does the physical size of a resistor give any indication to its rating?
Thanks for the help. Martin.
Approxinately 680 ohm is by what you need for 12V Connections to LED, you can use a 1K resistor and it will still work fine. LEDs are fine with 1/4 watt resistors. I don't think the color rings give you wattage, it's just the value and the tolerance.
When speaking of resistors which have coaxial leads (two long ones coming out the ends, not the surface mount type), the tiny ones are 1/4 watts and most ones you see around are 1/2 watt. So you should be ok, bit best to check when you buy.
Hope this helps a bit.
1KOhm, 1/4 Watt resistors are what I use for LED lights (Brown-Black-Red) for locomotives and signals, and have never had a problem with one not being bright enough.
Resistor size is an indicator of the Wattage rating -- the larger the physical size, the higher the Wattage. For an LED, 1/8 Watt or less would be sufficient, but are harder to find.
Measured with 12.2 VDC supply using 20 ma max current LED's. Values will vary a ittle depending on actual DC supply value.
Resistor Current
1000 ohm 9.0ma
750 ohm 12.45ma
680 ohm 13.12ma
510 ohm 16.25ma
A few people who use super bright LED's say they need to use more than 1k to get the briliance they like.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
The resistance required for an LED would depend on the voltage being applied and the current being limited. See the LED circuit. The resistance would be less for a 5V circuit than a 12 V circuit.
The power rating depends on both the voltage across a resistor and the current through it. Power can be calculated in a number of ways: P=V*I, P=V*V/R and P=I*I*R knowing that V=I*R. So for a 12V circuit with an LED having a voltage drop of 1.4 V the votlage across the resistor is 10.6V. if the current of the LED is 10 ma, then the power dissipated thru the resistor is 106mW. a 1/4W resistor would be more than sufficient even if you doubled the current. For a 5V circuit, the voltage drop across the resistor would be 3.6 and the power 36 mW. again, a 1/4 resistor is more than adequate.
Some resistors have color codes indicating both resistance and tolerance. i haven't seen any that indicator power.
greg - Philadelphia & Reading / Reading
The discussion so far has only addressed the single-LED configuration, which is the most common. Sometimes, particularly when we're talking about structure lighting, someone asks, "Can I just use one resistor for all the LEDs in a building?" Well, yes, but it's got to be the right resistor. If you set up a bunch of LEDs in parallel and connect them to a single resistor, the current through the resistor is the sum of the currents through all the LEDs. So, the power (proportional to the current) goes up in the same manner. Instead of that 1/4 watt resistor, you may need a full watt or more.
Resistors are cheap. Use one for each LED and you'll never have to worry about these things.
It takes an iron man to play with a toy iron horse.
With todays current crop of bright white LEDs, I've gone to a 2000 ohm resistor as a standard figure - and even that is still plenty bright ! Running them at their full potential will put a beam on the opposite wall in a dark room !
I've even used as high as a 42,000 ohm resistor on white LEDs used as class lights when I just need a nice "glow" to them.
For doing your mathematical calculations, white LEDs require a voltage typically of 3.4 volts at 20ma for their full potential.
Mark.
¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ
I have used three LED's in series using a 1k resistor with a 12 volt supply in a loco. With structure lighting, better to use a 12 vdc buss and a 1k with for each LED. Maybe in a single structure, three LED's on a single 1k resistor.
Now if you like dimming the lights, standard 12 volt bulbs with say a power pack for lighting.
You can probably dim LED's but I have never tried it in structures. I have experimented with dimming LED's at the workbench using a 1k resistor and variable 12 volt supply.
I have used LED's for power indication on DCC with a 1k resistor. Works fine. I found out a few years ago the NCE Power Cab uses a LED for power indication and a 1k resistor. A few people insist you need a diode for protection on DCC but have never had any issues.
Mark R. With todays current crop of bright white LEDs, I've gone to a 2000 ohm resistor as a standard figure - and even that is still plenty bright ! Running them at their full potential will put a beam on the opposite wall in a dark room ! I've even used as high as a 42,000 ohm resistor on white LEDs used as class lights when I just need a nice "glow" to them. For doing your mathematical calculations, white LEDs require a voltage typically of 3.4 volts at 20ma for their full potential. Mark.
I'm with Mark on this. More resistance is better. I rarely go lower than 2k on a headlight install -- and that's with my DCC system dialed down to 12.5 volts.
I also like to use lower voltage and lots of resistance to get nice dim bulbs to appear in my buildings. LEDs do have a narrow range of dimming, but when you get into that sweet spot you can do a lot with them. I also like to use cone/reflector LEDs, which look a lot like a bulb when dialed down. Some discussion down the page here: http://cs.trains.com/mrr/f/88/t/213765.aspx
Here's a couple of pics of the results you can achieve.
Mike Lehman
Urbana, IL
It all depends on the LED and also how it ends up mounting in the loco. If the LED is directly exposed, they almost certainly will be too bright without a larger than normal resistor. Some brands have better light bars than others - in my Proto 2K units, the YeloGlo LEDs I use aren;t too bright at 1K, not at all. In my Stewart/Bowser Baldwin switchers - 1K was WAY too bright.
As for reverse protection - LEDs in general have realtively low peak inverse voltage limits. A typical silicon rectifier diode is 50-400V, meaning it can block voltages up to that limit int he reverse direction. So if you are rectifying 12VAC, a 50V PIV diode is fine, if you are rectifying 120VAC, you want at least a 200V PIV diode. LEDs normally can;t take much reverse voltage - color LEDs, especially red, can take more, white LEDs can barely stand their forward voltage in reverse. Just looked at a few random datasheets, most LEDs are in the 5-6V range. Any more than that will break them down. A bi-color LED is two LEDs back to back, so the one not lit never sees more than the forward voltage of the other. Common practive where the direction can reverse is to add a regualr diode anti-parallel to the LED< thus when the current flow is reversed, the LED only sees about 0.7V across it in the reverse direction. I suspect that red LED pilot on the NCE PowerPro either has a diode like that across it or it's not actually connected to the DCC output side of the circuit, I cannot imagine a red LED lasting years with over 15V applied across it in the reverse direction on a regular basis, even with a low current.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Lots of good info, thanks everyone.
Martin:
One thing I would add is that you should determine what the maximum load is for the decoder lighting functions. If you happen to get into a situation where you are installing multiple LEDs in the locomotive you may need to use higher value resistors for each LED to limit the total load. I have an H16-44 with 14 LEDs (2 headlights, 2 in each number board =8, plus 4 marker lights). I used 2200 ohm resistors IIRC and everything works fine and the lights are plenty bright. Rich's chart in his post gives you a good example of current draw using various resistor values.
Dave
Edit:
Mark R. has gone to great lengths to show me that my assumption (based on decoder manuals but I can't put my finger on which ones) that you have to be aware of the total ma draw of the LEDs you wire to a decoder is entirely unfounded. Apparently you can hook up as many LEDs to a decoder as you want (or so) and you still won't let the smoke out.
I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!
Has anyone actually ever overloaded a decoder from having too many lights ? Don't think I've ever heard of it myself. Most decoders function outputs are rated between 200ma and 250ma.
Even if you ran the LEDs at their maximum potential (20ma) that's still 10 to 12 LEDs per function. Even on just a four function decoder, that would be between 40 and 48 LEDs ! That would be a total current draw of between 800ma and 960ma. Even at that, most decoders would still have at least an addition 250ma to 500ma for the motor available, which is still enough.
If anybody is running those kind of numbers in their LED installations, I'd love to see them !
Mark
Perhaps we have a different interpretation of the maximun rating for decoder outputs. I thought that the rating was cumulative, i.e. the decoder was only capable of providing so many ma regardless of the number of leads. If I am wrong I apologize. I still stand by my recommendation of using higher value resistors in multiple LED installations.
It IS cumulative - including the current draw of the motor.
Say for example a decoder is rated for 1 amp (many are rated higher). Under normal operation from my findings, an engine seldom draws more than half an amp (500ma). More often than not, my engines draw half that. That leaves 500ma for lighting. That's 125ma per function on a four function decoder (and they are usually rated higher than that). So you would still be looking at at least 24 LEDs at full rated power !
So even on the low side of decoder ratings, you should easily be able to run up to a couple dozen LEDs without any calculations required. And as I said, most decoders today are rated much higher. A loksound decoder will handle a peak of 2 amps with a 250ma rating on each of its six outputs. Pushed to the max, that would be 75 LEDs with 500ma left for the motor ! Granted, you are pushing the limits, but then, I don't think you could physically fit 75 LEDs in an engine shell and have much room left for anything else !
If you didn't run the LEDs at their maximum potential of 20ma, but rather run them at 10ma, you could run even more ! Anyone dare to try building the CP Rail Christmas train covered in a couple hundred 0402 SMT LEDs ?
OK Mark!
I guess I was wrong. I'll edit my post.
Now, about that CP Christmas train...... how many LEDs did you say?